This invention relates to a semiconductor device, and more particularly to a rectifying device for use with a rotary generator mounted on a vehicle.
In general, a rectifying device for use with a rotary generator mounted on a vehicle comprises a semiconductor chip having a rectifying function, a conductive lead electrode, a base electrode, and solder layers for bonding them together in laminated structure, wherein the peripheral portions of the semiconductor chip are hermetically sealed with such resin as silicone rubber.
When the rotary generator mounted on a vehicle operates, the semiconductor chip generates heat since large electric current flows through the rectifying device; and accordingly the semiconductor chip, the bonding solder layers, the lead electrode and the base electrode are heated up to at least 200° C. When the rotary generator stops operating, the current stops flowing and the rectifying device is cooled down to ambient temperatures. Since such a generator repeats cycles of operation and halt over a long period of time, the rectifying device undergoes the repetition of expansion due to heating and contraction due to cooling. If the thermal expansion coefficient of the semiconductor chip is different from those of the lead electrode and the base electrode, as is often the case, then thermal stress is generated in the solder layers for bonding them to each other. The thermal stress results in cracks due to thermal fatigue in the solder layers, the cracks increase in length and width, and finally the semiconductor device will be destroyed. U.S. Pat. No. 4,349,831 discloses a configuration which can reduce thermal stress in solder layers. This U.S. patent discloses a rectifying device wherein first and second stress relief members are provided between a semiconductor element and an electrode connected with a flexible wire lead and between the semiconductor element and a base member, the thermal expansion coefficient of each of the stress relief members being greater than that of the semiconductor element and smaller than those of the electrode and the base member, and wherein the first stress relief member is interposed between the base member and the semiconductor element and bonded to them by solder layers while the second stress relief member is interposed between the enlarged portion of the electrode and the semiconductor element and bonded to them by the solder layer.